The research program increases the sample of infant twins for a multi-method longitudinal appraisal of temperament from the neonatal period through 4 years of age. Infant twins offer a powerful resource for detecting similarities and differences among genetically related pairs and examining the stability and organization of temperament characteristics over age. From the perspective of developmental behavior genetics, features of temperament may be examined for genetic influences on dimensions of temperament and their reorganization when transitions occur. Twins provide an opportunity to detect environmental features of parents, family/home where two infants' temperament may differ markedly at a given time or over time; and they furnish a unique situation to observe infant-parent reciprocity when infant temperament may evoke different responses from parents. The principal aims of the program are to (a) complete a sample of neonatal assessments of all twins in newborn nurseries for an initial measure of irritability, reactivity, and developmental status; (b) complete recruiting a longitudinal sample of twins who will receive a multi-method appraisal of temperament at 3, 6, 9, 12, 18, 24, and 30 months, as based an direct observations of twins' behavior during standardized procedures in a structured laboratory setting, temperament questionnaires (Carey et al.) completed by parents, IBR ratings obtained during Bayley mental testing, and parental interviews; (c) complete home visits at 7 and 36 months for an appraisal of the family/home environment and its contribution to temperament development; (d) complete the sample of twins at 3 and 4 years engaged in structured standardized laboratory procedures designed to detect the links between temperament and social interaction (prosocial and agonistic features). All sessions are videotaped. The results are ultimately addressed to the questions of origins, patterning among variables, stability/instability over ages, and the Joint collaboration of genetic and environmental factors of determinants of temperament. This project, linked with other data sets, shows the relevance of newborn and infant temperament, examined longitudinally, for its contribution to preschoolers' liability for unintentional injuries, the development of aggression in early childhood, and the adjustment of children in the first grade at school.